We begin with a summary of handover procedures of [IEEE802.16e] and
then present the optimized complete FMIPv6 handover procedures by
using the link-layer handover indicators. The examples of handover
scenarios are described for both the predictive mode and reactive
mode.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document is to be interpreted as described in [RFC2119].
Most of terms used in this document are defined in MIPv6 [RFC3775]
and FMIPv6 [RFC5268].
The following terms come from the IEEE 802.16e specification
[IEEE802.16e].
MOB_NBR-ADV
An IEEE 802.16e neighbor advertisement message sent
periodically by a base station.
MOB_MSHO-REQ
An IEEE 802.16e handover request message sent by a mobile node.
MOB_BSHO-RSP
An IEEE 802.16e handover response message sent by a base
station.
MOB_BSHO-REQ
An IEEE 802.16e handover request message sent by a base
station.
MOB_HO-IND
An IEEE 802.16e handover indication message sent by a mobile
node.
BSID
An IEEE 802.16e base station identifier.

3. IEEE 802.16e Handover Overview
Compared with the handover in the WLAN (Wireless Local Area Network),
the IEEE 802.16e handover mechanism consists of more steps since the
802.16e embraces the functionality for elaborate parameter adjustment
and procedural flexibility.
When a mobile node (MN) stays in a link, it listens to the Layer 2
neighbor advertisement messages, named MOB_NBR-ADV, from its serving
base station (BS). A BS broadcasts them periodically to identify the
network and announce the characteristics of neighbor BSs. Receiving
this, the MN decodes this message to find out information about the
parameters of neighbor BSs for its future handover. With the
provided information in a MOB_NBR-ADV, the MN may minimize the
handover latency by obtaining the channel number of neighbors and
reducing the scanning time, or may select the better target BS based
on the signal strength, Quality-of-Service (QoS) level, service
price, etc.
The handover procedure is conceptually divided into two steps:
"handover preparation" and "handover execution" [SH802.16e]. The
handover preparation can be initiated by either an MN or a BS.
During this period, neighbors are compared by the metrics such as
signal strength or QoS parameters, and a target BS is selected among
them. If necessary, the MN may try to associate (initial ranging)
with candidate BSs to expedite a future handover. Once the MN
decides to handover, it notifies its intent by sending a MOB_MSHO-REQ
message to the serving BS (s-BS). The BS then replies with a
MOB_BSHO-RSP containing the recommended BSs to the MN after
negotiating with candidates. Optionally, it may confirm handover to
the target BS (t-BS) over backbone when the target is decided.
Alternatively, the BS may trigger handover with a MOB_BSHO-REQ
message.
After handover preparation, handover execution starts. The MN sends
a MOB_HO-IND message to the serving BS as a final indication of its
handover. Once it makes a new attachment, it conducts 802.16e
ranging through which it can acquire physical parameters from the
target BS, tuning its parameters to the target BS. After ranging
with the target BS successfully, the MN negotiates basic capabilities
such as maximum transmit power and modulator/demodulator type. It
then performs authentication and key exchange procedures, and finally
registers with the target BS. If the target BS has already learned
some contexts such as authentication or capability parameters through
backbone, it may omit the corresponding procedures. For the details
of the 802.16 handover procedures, refer to Section 6.3.22 of
[IEEE802.16e]. After completing registration, the target BS starts

to serve the MN and communication via target BS is available.
However, in case the MN moves to a different subnet, it should
reconfigure a new IP address and reestablish an IP connection. To
resume the active session of the previous link, the MN should also
perform IP layer handover.
4. Network Topology Acquisition and Network Selection
This section describes how discovery of adjacent networks and
selection of target network work in the IEEE 802.16e for background
information.
An MN can learn the network topology and acquire the link information
in several ways. First of all, it can do that via L2 neighbor
advertisements. A BS supporting mobile functionality shall broadcast
a MOB_NBR-ADV message periodically that includes the network topology
information (its maximum interval is 1 second). This message
includes BSIDs and channel information of neighbor BSs, and it is
used to facilitate the MN's synchronization with neighbor BSs. An MN
can collect the necessary information of the neighbor BSs through
this message for its future handover.
Another method for acquisition of network topology is scanning, which
is the process to seek and monitor available BSs in order to find
suitable handover targets. While a MOB_NBR-ADV message includes
static information about neighbor BSs, scanning provides rather
dynamic parameters such as link quality parameters. Since the
MOB_NBR-ADV message delivers a list of neighbor BSIDs periodically
and scanning provides a way to sort out some adequate BSs, it is
recommended that when new BSs are found in the advertisement, the MN
identifies them via scanning and resolves their BSIDs to the
information of the subnet where the BS is connected. The
association, an optional initial ranging procedure occurring during
scanning, is one of the helpful methods to facilitate the impending
handover. The MN is able to get ranging parameters and service
availability information for the purpose of proper selection of the
target BS and expediting a potential future handover to it. The
detailed explanation of association is described in Section 6.3.22 of
[IEEE802.16e].
Besides the methods provided by 802.16e, the MN may rely on other
schemes. For instance, the topology information may be provided
through the MIIS (Media Independent Information Service)
[IEEE802.21], which has been developed by the IEEE 802.21 working
group. The MIIS is a framework by which the MN or network can obtain
network information to facilitate network selection and handovers.

After learning about neighbors, the MN may compare them to find a BS,
which can serve better than the serving BS. The target BS may be
determined by considering various criteria such as required QoS,
cost, user preference, and policy. How to select the target BS is
not in the scope of this document.
5. Interaction between FMIPv6 and IEEE 802.16e
In this section, a set of primitives is introduced for an efficient
interleaving of the IEEE 802.16e and the FMIPv6 procedures as below.
The following sections present the handover procedures in detail by
using them.
o NEW_LINK_DETECTED (NLD)
A trigger from the link layer to the IP layer in the MN to
report that a new link has been detected.
o LINK_HANDOVER_IMPEND (LHI)
A trigger from the link layer to the IP layer in the MN to
report that a link-layer handover decision has been made and
its execution is imminent.
o LINK_SWITCH (LSW)
A control command from the IP layer to the link layer in the MN
in order to force the MN to switch from an old BS to a new BS.
o LINK_UP (LUP)
A trigger from the link layer to the IP layer in the MN to
report that the MN completes the link-layer connection
establishment with a new BS.
5.1. Access Router Discovery
Once a new BS is detected through reception of a MOB_NBR-ADV and
scanning, an MN may try to learn the associated access router (AR)
information as soon as possible. In order to enable its quick
discovery in the IP layer, the link layer (802.16) triggers a
NEW_LINK_DETECTED primitive to the IP layer (FMIPv6) on detecting a
new BS.
Receiving the NEW_LINK_DETECTED from the link layer, the IP layer
tries to learn the associated AR information by exchanging an RtSolPr
(Router Solicitation for Proxy Advertisement) and a PrRtAdv (Proxy
Router Advertisement) with the PAR (Previous Access Router).

According to [RFC5268], the MN may send an RtSolPr at any convenient
time. However, this proposal recommends that, if feasible, the MN
send it as soon as possible after receiving the NEW_LINK_DETECTED for
quick router discovery because detection of a new BS usually implies
MN's movement, which may result in handover.
Transmission of RtSolPr messages may cause the signaling overhead
problem that is mentioned in Section 2 of [RFC4907]. To rate-limit
the retransmitted RtSolPr messages, FMIPv6 provides a back-off
mechanism. It is also possible that attackers may forge a MOB_NBR-
ADV message so that it can contain a bunch of bogus BSIDs or may send
a flood of MOB_NBR-ADV messages each of which contains different
BSIDs. This problem is mentioned in Section 8.
5.2. Handover Preparation
When the MN decides to change links based on its policy such as the
degrading signal strength or increasing packet loss rate, it
initiates handover by sending a MOB_MSHO-REQ to the BS and will
receive a MOB_BSHO-RSP from the BS as a response. Alternatively, the
BS may initiate handover by sending a MOB_BSHO-REQ to the MN.
On receiving either a MOB_BSHO-RSP or a MOB_BSHO-REQ, the link layer
triggers a LINK_HANDOVER_IMPEND in order to signal the IP layer of
arrival of MOB_BSHO-REQ/MOB_BSHO-RSP quickly. At this time, the
target BS decided in the link layer is delivered to the IP layer as a
parameter of the primitive. The primitive is used to report that a
link-layer handover decision has been made and its execution is
imminent. It can be helpfully used for FMIPv6 as an indication to
start the handover preparation procedure, that is to send an FBU
(Fast Binding Update) message to the PAR.
To avoid erroneous results due to unreliable and inconsistent
characteristics of link, for instance, to move to the unpredicted
network or to stay in the current network after sending an FBU,
Section 2 of [RFC4907] advises the use of a combination of signal
strength data with other techniques rather than relying only on
signal strength for handover decision. For example, the
LINK_HANDOVER_IMPEND may be sent after validating filtered signal
strength measurements with other indications of link loss such as
lack of beacon reception.
Once the IP layer receives the LINK_HANDOVER_IMPEND, it checks
whether or not the specified target network belongs to a different
subnet based on the information collected during the Access Router
Discovery step. If the target proves to be in the same subnet, the
MN can continue to use the current IP address after handover, and
there is no need to perform FMIPv6. Otherwise, the IP layer

formulates a prospective NCoA (New Care-of Address) with the
information provided in the PrRtAdv message and sends an FBU message
to the PAR.
When the FBU message arrives in the PAR successfully, the PAR and the
NAR (New Access Router) process it according to [RFC5268]. The PAR
sets up a tunnel between the PCoA (Previous Care-of Address) and NCoA
by exchanging HI (Handover Initiate) and HAck (Handover Acknowledge)
messages with the NAR, forwarding the packets destined for the MN to
the NCoA. The NCoA is confirmed or re-assigned by the NAR in the
HAck and, finally delivered to the MN through the FBack (Fast Binding
Acknowledgment) in case of predictive mode.
After the MN sends a MOB_HO-IND to the serving BS, data packet
transfer between the MN and the BS is no longer allowed. Note that
when a MOB_HO-IND is sent out before an FBack arrives in the MN, it
is highly probable that the MN will operate in reactive mode because
the serving BS releases all the MN's connections and resources after
receiving a MOB_HO-IND. Therefore, if possible, the MN should
exchange FBU and FBack messages with the PAR before sending a MOB_HO-
IND to the BS so as to operate in predictive mode.
5.3. Handover Execution
If the MN receives an FBack message on the previous link, it runs in
predictive mode after handover. Otherwise, it should run in reactive
mode. In order for the MN to operate in predictive mode as far as
possible after handover, implementations may allow use of a
LINK_SWITCH primitive. The LINK_SWITCH is a command in order to
force the MN to switch from an old BS to a new BS and the similar
concept has introduced for the wireless LAN in [RFC5184]. When it is
applied, the MN's IP layer issues a LINK_SWITCH primitive to the link
layer on receiving the FBack message in the previous link. Until it
occurs, the link layer keeps the current (previous) link if feasible
and postpones sending a MOB_HO-IND message while waiting for the
FBack message.
After switching links, the MN synchronizes with the target BS and
performs the 802.16e network entry procedure. The MN exchanges the
RNG-REQ/RSP, SBC-REQ/RSP, PKM-REQ/RSP, and REG-REQ/RSP messages with
the target BS. Some of these messages may be omitted if the
(previously) serving BS transferred the context to the target BS over
the backbone beforehand. When the network entry procedure is
completed and the link layer is ready for data transmission, it
informs the IP layer of the fact with a LINK_UP primitive.

Section 2 of [RFC4907] recommends that link indications should be
designed with built-in damping. The LINK_UP primitive defined in
this document is generated by the link layer state machine based on
the 802.16e link layer message exchanges, that is, the IEEE 802.16e
network entry and the service flow creation procedures. Therefore,
the LINK_UP is typically less sensitive to changes in transient link
conditions. The link may experience an intermittent loss. Even in
such a case, the following FMIPv6 operation is performed only when
the MN handovers to the link with a different subnet and there is no
signaling overhead as a result of a intermittent loss.
5.4. Handover Completion
When the MN's IP layer receives a LINK_UP primitive from the link
layer, it should check whether it has moved into the target network
predicted by FMIPv6. In case the target BS is within the same
subnet, the MN does not perform the FMIPv6 operation.
* If the MN discovers itself in the predicted target network and
receives an FBack message in the previous link, the MN's IP
layer sends an UNA (Unsolicited Neighbor Advertisement) to the
NAR (predictive mode).
* If the MN has moved to the target network without receiving an
FBack message in the previous link, the IP layer sends an UNA
and also an FBU message immediately after sending the UNA
message (reactive mode). The NAR may provide a different IP
address by using an RA (Router Advertisement) with a NAACK
(Neighbor Advertisement Acknowledge) option other than the
formulated NCoA by the MN.
* The MN may discover itself in the unpredicted network
(erroneous movement). If this is the case, the MN moves to the
network that is not the target specified in the
LINK_HANDOVER_IMPEND primitive. For the recovery from such an
invalid indication, which is mentioned in Section 2 of
[RFC4907], the MN should send a new FBU to the PAR according to
Section 5.6 of [RFC5268] so that the PAR can update the
existing binding entry and redirect the packets to the new
confirmed location.
In both cases of predictive and reactive modes, once the MN has moved
into the new link, it uses the NCoA formulated by the MN as a source
address of the UNA, irrespective of NCoA availability. It then
starts a Duplicate Address Detection (DAD) probe for NCoA according
to [RFC4862]. In case the NAR provides the MN with a new NCoA, the
MN MUST use the provided NCoA instead of the NCoA formulated by the
MN.

When the NAR receives an UNA message, it deletes its proxy neighbor
cache entry if it exists, and forwards buffered packets to the MN
after updating the neighbor cache properly. Detailed UNA processing
rules are specified in Section 6.4 of [RFC5268].
6. The Examples of Handover Scenario
In this section, the recommended handover procedures over 802.16e
network are shown for both predictive and reactive modes. It is
assumed that the MN handovers to the network that belongs to a
different subnet.
In the following figures, the messages between the MN's Layer 2 (MN
L2) and the BS are the IEEE 802.16 messages, while messages between
the MN's Layer 3 (MN L3) and the PAR and messages between PAR and NAR
are the FMIPv6 messages. The messages between the MN L2 and the MN
L3 are primitives introduced in this document.
6.1. Predictive Mode
The handover procedures in the predictive mode are briefly described
as follows. Figure 3 illustrates these procedures.
1. A BS broadcasts a MOB_NBR-ADV periodically.
2. If the MN discovers a new neighbor BS in this message, it may
perform scanning for the BS.
3. When a new BS is found through the MOB_NBR-ADV and scanning,
the MN's link layer notifies it to the IP layer by a
NEW_LINK_DETECTED primitive.
4. The MN tries to resolve the new BS's BSID to the associated
AR by exchange of RtSolPr and PrRtAdv messages with the PAR.
5. The MN initiates handover by sending a MOB_MSHO-REQ message
to the BS and receives a MOB_BSHO-RSP from the BS.
Alternatively, the BS may initiate handover by sending a
MOB_BSHO-REQ to the MN.
6. When the MN receives either a MOB_BSHO-RSP or a MOB_BSHO-REQ
from the BS, its link layer triggers a LINK_HANDOVER_IMPEND
primitive to the IP layer.

7. On reception of the LINK_HANDOVER_IMPEND, the MN's IP layer
identifies that the target delivered along with the
LINK_HANDOVER_IMPEND belongs to a different subnet and sends
an FBU message to the PAR. On receiving this message, the
PAR establishes tunnel between the PCoA and the NCoA by
exchange of HI and HAck messages with the NAR, and it
forwards packets destined for the MN to the NCoA. During
this time, the NAR may confirm NCoA availability in the new
link via HAck.
8. The MN receives the FBack message before its handover and
sends a MOB_HO-IND message as a final indication of handover.
Issue of a MOB_HO-IND may be promoted optionally by using a
LINK_SWITCH command from the IP layer. Afterwards it
operates in predictive mode in the new link.
9. The MN conducts handover to the target BS and performs the
IEEE 802.16e network entry procedure.
10. As soon as the network entry procedure is completed, the MN's
link layer signals the IP layer with a LINK_UP. On receiving
this, the IP layer identifies that it has moved to a
predicted target network and received the FBack message in
the previous link. It issues an UNA to the NAR by using the
NCoA as a source IP address. At the same time, it starts to
perform DAD for the NCoA.
11. When the NAR receives the UNA from the MN, it delivers the
buffered packets to the MN.

7. IEEE 802.21 Considerations
It is worth noting that great research has been conducted on defining
generic services in the IEEE 802.21 working group that facilitate
handovers between heterogeneous access links. The standard works are
named as a Media Independent Handover (MIH) Service [IEEE802.21], and
propose three kinds of services: Media Independent Event Service
(MIES), Media Independent Command Service (MICS), and Media
Independent Information Service (MIIS).
An MIES defines the events triggered from lower layers (physical and
link) to higher layers (network and above) in order to report changes
of physical and link-layer conditions. On the other hand, an MICS
supports the commands sent from higher layers to lower layers, and it
provides users with a way of managing the link behavior relevant to
handovers and mobility. An MIIS provides a framework by which the MN
or network can obtain network information to facilitate network
selection and handovers.
Although the purpose of IEEE 802.21 has been developed to enhance the
user experience of MNs roaming between heterogeneous networks, the
results may be utilized to optimize the handover performance in a
homogeneous network. When the MIH primitives are available for
handover in the 802.16e network, the MN can use them instead of the
primitives proposed in this document. Table 1 shows examples of the
mapping between the proposed primitives and the MIH primitives.
+-------------------------+-------------------------+
| Proposed primitives | MIH primitives |
+===================================================+
| NEW_LINK_DETECTED | LINK_DETECTED |
+---------------------------------------------------+
| LINK_HANDOVER_IMPEND | LINK_HANDOVER_IMMINENT |
+---------------------------------------------------+
| LINK_SWITCH | HANDOVER_COMMIT |
+---------------------------------------------------+
| LINK_UP | LINK_UP |
+---------------------------------------------------+
Table 1. The Proposed Primitives and MIH Primitives8. Security Considerations
The primitives defined in this document are used only for local
indication inside of the MN, so no security mechanism is required to
protect those primitives. However, FMIPv6 messages and IEEE 802.16e
messages, which may trigger the primitives, need to be protected.

Security considerations of the FMIPv6 specification [RFC5268] are
applicable to this document. It is also worthwhile to note that the
IEEE802.16e has a security sub-layer that provides subscribers with
privacy and authentication over the broadband wireless network. This
layer has two main component protocols: a privacy key management
protocol (PKM) for key management and authentication and an
encapsulation protocol for encrypting data. From the perspective of
the 802.16e, FMIPv6 messages are considered as data and are delivered
securely by using those protocols.
However, some of IEEE 802.16e management messages are sent without
authentication. For example, there is no protection to secure
802.16e broadcast messages. It may be possible for the attacker to
maliciously forge a MOB_NBR-ADV message so that it contains the bogus
BSIDs, or send a flood of MOB_NBR-ADV messages having different bogus
BSIDs toward the MN. As a result, the MN may trigger a bunch of
NEW_LINK_DETECTED primitives and send useless consecutive RtSolPr
messages to the PAR, finally resulting in wasting the air resources.
Therefore, the MN SHOULD perform scanning when detecting new BSs in
the received MOB_NBR-ADV messages in order to assure the included
neighbor information.
It is also possible that attackers try a DoS (Denial-of-Service)
attack by sending a flood of MOB_BSHO-REQ messages and triggering
LINK_HANDOVER_IMPEND primitives in the MN. But the IEEE 802.16e
provides a message authentication scheme for management messages
involved in handover as well as network entry procedures by using a
message authentication code (MAC) such as HMAC/CMAC (hashed/cipher
MAC). Thus, those management messages are protected from the
malicious use by attackers who intend to trigger LINK_HANDOVER_IMPEND
or LINK_UP primitives in the MN.
9. Acknowledgments
Many thanks to the IETF Mobility Working Group members of KWISF
(Korea Wireless Internet Standardization Forum) for their efforts on
this work. In addition, we would like to thank Alper E. Yegin,
Jinhyeock Choi, Rajeev Koodli, Jonne Soininen, Gabriel Montenegro,
Singh Ajoy, Yoshihiro Ohba, Behcet Sarikaya, Vijay Devarapalli, and
Ved Kafle who have provided technical advice.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.